In recent years, as electronic technology advances, making electronic devices more compact and lightweight has been rapidly advancing. Particularly, due to the spread and increased functions of portable electronic devices such as portable telephones or notebook computers, as the portable power supplies of these devices, there is a demand for development of secondary batteries that have high energy density, are compact and lightweight. To satisfy such a demand, lithium-ion secondary batteries, which are non-aqueous electrolyte secondary batteries are frequently used. Lithium-ion batteries are not limited to this kind of use, and research and development with the aim at using such batteries as a large power source for hybrid automobiles and electric automobiles is also advancing. As the cathode active material of lithium-ion secondary batteries, a lithium-cobalt composite oxide (LiCoO2) which can be relatively readily synthesized is mainly used.
However, expensive and rare cobalt compounds are included in the raw material of the lithium-cobalt composite oxide. Therefore, the cost per volume of a lithium-cobalt secondary battery that uses a lithium-cobalt composite oxide as the cathode material can reach up to approximately four times that of a nickel-metal hydride battery. Due to this high cost, at the present time the uses of lithium-ion batteries are rather limited. Consequently, lowering the cost of cathode active material and making it possible to provide less expensive lithium-ion secondary batteries has very large significance for making current portable electronic device more lightweight and compact.
A lithium-manganese composite oxide (LiMn2O4) that uses manganese that is less expensive than cobalt and lithium-nickel composite oxide (LiNiO2) that uses nickel are being investigated as cathode active material for a lithium-ion battery that can replace lithium-cobalt composite oxide.
Of these, lithium-manganese composite oxide is a material having inexpensive manganese as a raw material, and is a safe material that has excellent heat stability. However, the theoretical capacity is only about half that of a lithium-cobalt composite oxide, so has a disadvantage in that it would be difficult to answer the need for ever increasing high capacity of a lithium-ion secondary battery. Moreover, at temperatures of 45° C. or greater, there is a disadvantage in that self discharge is extreme, and there is decreased charge and discharge life.
On the other hand, lithium-nickel composite oxide, similar to manganese, can be obtained inexpensively and stably, and has a higher capacity when compared with lithium-cobalt oxide, so is expected to become mainstream as the next generation cathode active material, so much research and development is actively being advanced. However, a lithium-ion secondary battery that uses a lithium-nickel composite oxide composed of only lithium and nickel as the cathode active material has a problem of inferior cycling characteristics when compared with using a lithium-cobalt composite oxide. This is because in a lithium-nickel composite oxide, as the lithium separates, the crystal structure between hexagonal crystals and monoclinic crystals changes (phase transition), and due to the lack of reversibility of that change, as the charging and discharging reaction is repeated, the sites where lithium can separate and be inserted are gradually lost.
In order to solve this problem, replacing part of the nickel with cobalt is being studied. By this replacement, the crystal structure of the lithium-nickel composite oxide is stabilized, and phase transition of the crystal structure as lithium is separated is suppressed. In this case, as the amount of replacement cobalt increases, the crystal structure becomes more stable and the cycling characteristics are improved, however, there is a problem of an increase in cost. In order to obtain the effect above while at the same time keeping the amount of replacement cobalt small, uniformly dispersing cobalt on an atomic level into a nickel composite hydroxide precursor is effective.
As a method for making it possible to uniformly disperse cobalt is a reactive crystallization method. For example, JP H09-270258 (A) discloses a continuous crystallization method in which a nickel salt aqueous solution, a cobalt salt aqueous solution and a caustic alkali aqueous solution are continuously supplied into a reaction vessel in which the pH value and temperature are adjusted, while keeping the concentration and flow rate controlled, and by collecting the product from the reaction aqueous solution that overflows from the reaction vessel, the characteristics of the obtained nickel-cobalt composite hydroxide are controlled. With this kind of continuous crystallization method, stability of the crystal structure of the nickel-cobalt composite hydroxide is improved, and phase transition due to charging and discharging is suppressed. In addition, the crystal particle boundaries that are a cause of the breakdown of particle structure become very few, and it is possible to prevent the particles from becoming minute and falling off, so it is possible to obtain a cathode active material that has good cycling characteristics.
However, when producing nickel-cobalt composite hydroxide with this kind of continuous crystallization method, the particle size distribution becomes a normal distribution and spreads easily, so it become very difficult to obtain particles having a uniform particle size. Using a cathode active material having particles with a wide particle size distribution causes uneven voltage to be applied to particles inside an electrode, and as charging and discharging is repeated, minute particles selectively deteriorate, and the capacity of the lithium-ion battery decreases. Therefore, in the continuous crystallization method described above, it is not possible to sufficiently improve cycling characteristics of the cathode active material.
In order to obtain particles having a sharp particle size distribution, a batch method is more useful than a continuous crystallization method, however, a batch method has a disadvantage in that productivity is inferior to a continuous crystallization method. Particularly, when trying to obtain large particles that are 10 μm or more by a batch method, it is necessary to increase the amount of raw material supplied, however, in order to do so a large reaction vessel must be used, and productivity becomes even worse.
For these reasons, even for the case of a continuous crystallization method, development of a method for obtaining a nickel composite hydroxide having a sharp particle distribution is being pursued. For example, JP H10-265225 (A) and JP 2003-086182 (A) disclose technology for recovering particles produced by a continuous crystallization method while performing classification. More specifically, a classifying system is disclosed in which by constructing the reaction vessel so as to have a main body and a separation apparatus that is integrally provided on the bottom side of the main body, particles that are grown inside the main body and whose specific gravity have increased are collected and recovered by the separation apparatus that is provided on the bottom side of the main body, and underdeveloped particles are pushed back into the main body by the upward flow inside the separation apparatus. With this kind of method, even in the case of a continuous crystallization method, it is possible to obtain particles having a sharp particle size distribution. However, with this technology, it is necessary to strictly manage the crystallization conditions, so application to production on an industrial scale is difficult, and because the reaction process and separation process are performed in one reaction, there is a possibility that underdeveloped particles will be mixed in, and there are limits to being able to make the recovered particles uniform.